Subgrid Surface Fluxes in Fair Weather Conditions during TOGA COARE: Observational Estimates and Parameterization

Abstract

Bulk aerodynamic formulas are applied to meteorological data from low-altitude aircraft flights to obtain observational estimates of the subgrid enhancement of momentum, sensible heat, and latent heat exchange at the atmospheric?oceanic boundary in light wind, fair weather conditions during TOGA COARE (Tropical Ocean Global Atmosphere Coupled Ocean?Atmosphere Response Experiment). Here, subgrid enhancement refers to the contributions of unresolved disturbances to the grid-box average fluxes at the lower boundary of an atmospheric general circulation model. The observed subgrid fluxes increase with grid-box area, reaching 11%, 9%, 24%, and 12% of the total sensible heat, latent heat, scalar wind stress, and vector wind stress magnitude, respectively, at a grid-box size of 2° ? 2° longitude and latitude. Consistent with previous observational and modeling studies over the open ocean, most of the subgrid flux is explained by unresolved directional variability in the near-surface wind field. The authors find that much of the observed variability in the wind field in the presence of fair weather convective bands and patches comes from contributions of curvature and speed variations of simple larger-scale structure across the grid box. Inclusion of a grid-scale-dependent subgrid velocity scale in the bulk aerodynamic formulas effectively parameterizes the subgrid enhancement of the sensible heat flux, latent heat flux, and vector stress magnitude, and to a lesser degree the subgrid enhancement of the scalar wind stress. An observational estimate of the subgrid velocity scale derived from one-dimensional aircraft flight legs is found to be smaller than that derived from a two-dimensional grid-box analysis. The additional enhancement in the two-dimensional case is caused by the nonhomogeneous and nonisotropic characteristics of the subgrid-scale wind variability. Long time series from surface-based platforms in the TOGA COARE region suggest that measures of convective activity, in addition to geometric grid-scale parameters, will be required to more accurately represent the subgrid velocity scales.